Electrodynamic loud-speaker



March 14, 1933; ELSPRDIDHAM 1,901,331

ELECTRDYNAMIC LOUD SPEAKER filed May 15, 1929' IN V EN TOR. Alldfiaon/ TTORNEYS.

Patented Mar. 14, 1933 UNITED STATES- PATENT OFFICE EDWIN S. PBIDH AI, OF OAKLAND, CALIFORNIA, ASSIGNDR TO THE MAGNAVOX GOM-v PALNY, OF OAKLAND, CALIFORNIA, A CORPORATION OF ARIZONA.

- ELECTRODYNAMIC LOUD-SPEAKER Applicationflled Kay 15,

My present invention relates to a loud speaker unit, and especially to aunit that operates on the electro-dynamic principle.

Such units are now well known. They. include a magnetic structure providing a dense magnetic field of substantially constant value and a conductor, capable of movement transversely of the field, which is energized with current that carries the signaling impulses. The reactionbetween the field and the current in the conductor causes the conductor to move in accordance with.

the signaling impulses, and thereby to vibrate an air actuator, such as a cone 'or diaphragm.

These devices operate very satisfactoril when the magnetic field is obtained wit the aid of an electro-magnet energized by a constant potential source of electromotive force, such as a battery. If, however, it is attempted to provide a magnetic field by using a fluctuating current, such as is obtained by the rectification of an alternating current, it is found that these fluctuations are apt to cause annoying extraneous sounds in the loud speaker. In late years, the desirability of operating dynamic speakers from rectified current has been recognized, especially in connection with so-called elec trlcally operated radio sets. Hence, it is.

urgent that some scheme be provided for eliminating the effects of the fluctuations.

It is, accordingly, one of the objects of my invention to make it possible, in a simple and inexpensive manner, to overcome these disadvantages due to the fluctuations.

In a prior patent, Number 1,105,924, issued to me and Peter L. Jensen, as joint inventors, there is disclosed a means for neutralizing fluctuations in the electromotive force used in magnetizing anelectro-dynamic telephone. 11 this patent, however, the neutralizing coil is shown as mounted upon a separate core which is independently energized by an additional electro-magnetic coil and the arrangement is such that the.

neutralizing coil can be moved for the pur'-' pose of varying its efiectiveness. This arrangement, as will be readily appreciated, is somewhat expensive and cumbersome 1929. serial No. 363,311.

whenembodied in present day types of apparatus and it is, therefore, a further object of my invention to provide an improved neutralizing means and arrangement of apparatus which is particularly adaptable to present day devices.

My invention possesses many other advantages, and has other objects which may be made more easily apparent from a consideration of one embodiment of my invention. For this purpose ,I have shown all embodiment of my invention in the drawing which accompanies and forms a part of the present specification. I shall now pros ceed to describe this embodiment in detail, as it fully illustrates the general principles of my invention.

Referrin to the drawing:- The sing e figure is a diagrammatic view, showing my invention as a plied to a loud speaker, the structure of w 10b is diagrammatically illustrated in section.

'The "magnetic circuit that produces the magnetic field is formed partly by a casing exciting coil 15 disposed on the pole piece 13, whereby radial lines of magnetic flux are produced across the annular air gap at the end of the pole 13. Thecoil 15 is shown as indirectly energized from a source of alternating current, such as the primary coil 16' of a transformer. The secondary coil 17 is in series with the coil 15; and a rectifier is also included in the coil circuit.

Such a rectifier can be a vaccum tube device 18, or it can be of. any other desired type that is capable of passing a suificient current through it to the coil 15.

The signaling impulses are impressed upon an annular voice or modulated current coil 19 disposed in the annular air gap. This coil is mechanically connected to an air actuator, as for example, to the small end of a light, rigid cone 20, the edge of which is appropriately supported.

It is evident that, without further appa-- ratus, the coil 19 will be axially urged with a force depending on the current flowing in the coil 19, and on the intensity of the magnetic field in the annular gap. If this intensity varies due to the current fluctuations in coil 15, then -a movement is imparted to coil 19 that is superposed upon the movement due to the signaling impulses.

In order to neutralize the effect of these fluctuations, I provide a fixed or stationary coil 21 that is disposed on pole piece 13 adiacent the annular gap, and in series with the voice coil 19. This coil 21 is wound with rather large wire to keep its resistance low. It is held in place against a shoulder 22 on the pole piece 13 formed by upsetting the material. The shoulder 22 also serves to hold the coil 15 in place.

Since the function of the coil 21\is to neutralize the efiect of field current variations on the voice or diaphragm actuating coil 19 it must be located at a point whereit will be in inductive relation with the field coilvoice coil 19 in the air gap, be encircled by more of the total flux of the magnetic field than will be the voice coil 19 and consequently it may be said to be more sensitive to field current changes than the voice coil.

As a-result of the above suggested difi'er- .ence in sensitivity the neutralizing coil 21 is generally provided with a fewer number of turns than the voice or diaphragm coil 15. When the proper number of turns is provided in the neutralizing coil 21 it is possible to completely eliminate all hum in the apparatus due to variations in the exciting current flowing through the field coil 15 and at the same time not interfere with the free movement of the diaphragm actuating coil 19 in response to the modulated signal current impressed thereupon.

If we assume that a fluctuation produces a current in the coil 15, this'fluctuation will also induce an electromotive force in the neutralizing coil 21'; and when this coil 21 is connected in series circuit and opposed to the voice coil 19, the E. M. F. induced therein will oppose the electromotive force induced in the coil 19 due to this fluctuation. From this, it will be seen that by a proper choice of relative number of turnsin coils 19 and 21, these opposingefi'ects can be fairly; well balanced, so that the. ultimate result will be a substantial neutralization of the disturbing efi'ects due to variations in the rectified current for the complete range of operation.

Recapitulating, when the neutralizing coil 21 is connected in series with the moving or voice coil 19 and the secondary 17 of the coupling transformer, as illustrated, the neutralizing coil being wound about the pole piece 13 of the electro-magnet, it will be seen, since the rectifier. 18 supplies to the electro-magnet coil 15 a pulsating current, that the magnetic flux roduced by the coil 15 will also fluctuate. ince the moving coil is in the field of this flux therewill be a reaction between it and the magnetic flux and, as a'result, the voice coil 19 will tend to move, its movements having the same frequency as the fluctuations in the field current. If the diaphragm moves, however, sound will be produced and, as a result, we would get an audible hum. With my invention, the effect of the pulsating field current is, however, nullified or neutralized by the neutralizing coil 21, which coil is also in the magnetic field produced by the coil 13, and

it, therefore, has induced in it a voltage cor-- responding in frequency to the pulsating magnetic field energizing current. The voltage thus induced in the neutralizing coil will send a current around the circuit consisting of the transformer, the moving voice coil, and the neutralizing coil. VVlth this arrangement, the magnitude of this induced current will be such that its effect on the moving voice coil will be equal and opposite to that produced directly on the moving coil by the flux of the magnetic field, and since the two effects are equal and opposite, they will neutralize each other and prevent the diaphragm 20 from being actuated due to variations in the energizing electromotive force and thus producing a hum corresponding thereto.

While I have shown the preferred form of my invention as now known to me, it is to be understood that various changes may be made in its construction without departing from the spirit of the invention as defined in the appended claims.

Having thus described my invention, what I claim and desire to secure by Letters Patent is 1..In a signal reproducer, means for electro-magnetically producing a magnetic field, a diaphragm, a conductor for actuating said diaphragm capable of being energized from an external source of modulated current and moving transversely to said field, and a sec- 0nd stationary conductor connected 1n series structure having an air ga a coil for magnetizing said magnetic eld structure, a diaphragm, a voice current carryin conductor capable of moving transverse y 'to the magnetic field in the air gap and actuating said diaphragm, and a stationary coil connected in opposed series circuit .with said voice current conductor and inductively re lated to said magnetic circuit for neutralizing currents induced in said conductor by variations in the strength of said magnetic circuit.

3. In a signal reproducer, a magnet structure having a pole piece and an air ga between the pole piece and the rest 0 the structure, a diaphragm actuating conductor having connections with'a source of modulated current of sound producing frequend capable of transverse movement in said air gap, a. coil for energizing the magnet structure to provide a magnetic flux across the air gap, and a stationary coil connected in series circuit with said conductor and with said source of modulated current disposed uponathe pole piece near the air gap in inductive relation with the coilwhich energizes the magnet structure.

4. In a signal reproducer, a magnetic structure, a pole piece in said structure and forming therewith an airgap, a coil on said pole piece for magnetizing the air gap, a diaphragm actuating conductor adapted to be energized by a modulated current of sound producing frequency and move transversely in said air gap, and a stationary coil in series circuit with said diaphragm actuating conductor and disposed on the pole piece adjacent the air gap, said stationary coil being in inductive relation with the coil that magnetizes the air gap.

5. In a loud speaker of the electro-dynamic type, the combination of a magnetic,

field structure-having an'outer metallic casing and a centrally disposed core mounted therein, a winding upon said core for energizing said magnetic field structure, an annular pole piece for closing said magnetic field structure and forming an annular air gap at the end of said core, a voice current carrying coil disposed in said annular air gap adapted to move therein and actuate a sound reproducing diaphragm, and a second coil. mounted upon said core connected in series with said voice currentcarrying coil for neutralizing currents induced in said voice coil by reason of fluctuations in the current flowing in said magnetic field energizing. coil. i

6. In a loud speaker of the electro-dy namic type, the combination of a magnetic field structure having an outer metallic casing and a centrally disposed core mounted. \therein, a primary winding upon said core for energizing said magnetic field structure 03 adapted to be excited by a rectified alternating current, an annular pole piece for closing said magnetic field structure and form- .ing an annular air gap at the end of said core, a coil disposed in said annular air gap adapted to be energized from an external source by a modulated voice current and actuate a sound reproducing diaphragm, and a secondary winding mounted upon said core adjacent said primary winding connected in series with said diaphragm actuating coil for neutralizing currents induced in said diaphragm actuating coil by reason of fluctuations in the current flowing in said primary winding.

7. In a loud speaker of the electro-dynamic type, the combination of a magnetic field structure having a centrally disposed core mounted therein, a winding upon said core for energizing said magnetic field structure, an annular pole piece'associated with said magnetic field structure and forming an annular air gap'at the end of said core, a voice current carrying coil disposed in said annular air gap adapted to move therein and actuate a sound reproducing diaphragm, and a second and stationary coil mounted upon -said core connected in opposed series relation with said moving diaphragm actuating coil for neutralizing currents induced in said diaphragm actuating coil by reason of fluctuations in the current flowlng in said magnetic field energizing coil.

8. In a signal reproducer the combination of a magnetic field structure, a diaphragm actuating coil having connections with an external source of modulated current of sound producing frequency disposed inoperative relation with said field structure, and a stationary coil disposed in inductive relation with said'field structure and connected to impress an electromotive force upon said diaphragm actuating coil equal and opposite to the electromotive force generated in said latter coil b variations in the flux of said magnetic fiel 9. In a sound reproducing system, the combination of a field structure having an air-gap, a field coil upon said field structure for producing a magnetic flux across 'said air-gap, a source of rectified alternating current for energizing said field coil, a modulated current carrying coil disposed in said air-gap adapted to actuate a diaphragm to produce sound, a source of modulated current for said last coil, and a neutralizing coil disposed in inductive relation with said field coil and connected in series circuit with said modulated current carrying coil for preventing the reproduction of sound due to fluctuations in the rectified current which energizes said field coil.

10. In a loud speaker apparatus, a mag netic circuit providing an air gap, a modulated current coil extending into the air gap and movable therein, and a fixed neutralizing coil conductively connected to the modulated current coil and disposed in inductive relation with the magnetic circuit to neutralize the effect of induced currents in the modulated current coil.

11. In a loud speaker apparatus, a magmodulated current coil carried by the sound producing member, and a fixed neutralizing coil associated with the means for energizing the magnetic circuit for generating a current for neutralizing currents induced in the modulated current coil.

13. In a loud speaker apparatus, a magnetic circuit comprising a cup-shaped mem-- her, a central pole piece mounted therein, an

annular pole piece secured to the cup-shaped member and forming an air gap with the central pole piece, a diaphragm for reproducing sounds, a modulated current coil carried thereby and extending into the air gap, and a neutralizing coil wound upon the central pole piece for generatinga neutralizing current for the modulated current coil to prevent movement of the diaphragm due to flux changes in the air gap.

14. In a loud speaker apparatus, a magnetic circuit provlding an air gap, a modulated current coil extending into the air gap, a stationary neutralizing coil in series with the modulated current coil connected thereto in such a direction that the current generated in the neutralizing .coil and flowing through the modulated current coil is opposite in direction to induced currents in the modulated current coil caused by flux variav tions in the magnetic circuit.

15. In a loud speaker apparatus, a magnetic circuit having an air gap, a field coil for energizing said circuit, an alternating current rectifier for energizing the field coil, a diaphragm for reproducing sounds, a cylinder carried by said diaphragm having a portion extending into the air gap, a modulated current coil carried by the cylinder and extending into the air gap, a neutralizing coil wound upon the central pole piece, and means for interconnecting the neutralizing coil and the modulated current coil, where by a current is caused to flow in the modulated current coil which is equaland opposite to the current induced in the modulated current coil by variations-in the flux of the magnetic circuit. 1

16. In a loud speaker apparatus,- a magnetic circuit having an air gap, means for supplying an energizing current therefor, a modulated current coil disposed in said air gap, and means for eliminating the effect of variations of the energizing current upon said modulated current coil comprising a stationary neutralizing coil inductivel related to the magnetic circuit and di erentially connected to said modulated. current coil.

17. In a loudspeaker apparatus, a magnetic circuit, means for energizing the magnetic circuit, a sound producing member, a modulated current coil carried by the sound producing member, and a neutralizin coil differentially connected in series wit the modulated current coil and .more subject to flux variations in the magnetic circuit than the modulated current coil and associated with the magnetic circuit for generating a neutralizing current for the modulated current coil.

18. In a loud speaker apparatus, a magnetic circuit, means for supplying an energizing current therefor, a voice current coil disposed in operative relation with said magnetic circuit, and means for eliminating the effect of variations of the energizin current comprising a neutralizing coil di erentially connected to said voice current coil, said neutralizing coil being more subject to change of flux in the magnetic circuit than the voice current coil.

EDWIN S. PRIDHAJH. 

